Small protein molecules known as neuropeptides may function as neurotransmitters or "neuromodulators" in the brain and spinal cord. In order to understand more about the anatomical basis of their role in health and disease, the cells containing them should be studied with the electron microscope. Numerous technical problems have hindered this work, the most important one being that the neuropeptides are largely dissolved out of the tissue by the usual methods of preparation for electron microscopy. We have used freeze-drying methods for electron microscopy to circumvent this problem, and have successfully stained ultrathin serial sections for six neuropeptides, neurophysin, enkephalin, somatostatin, neurotensin, substance P, and alpha-MSH. We envision being able to correlate the presence of certain peptides with structures that are anatomically identifiable with the electron microscope by comparing electron micrographs of sections immunocytochemically stained for the peptides with adjacent sections that have been stained to reveal normal morphology. We also plan to determine if the peptides are individually contained in anatomically discrete compartments, such as nerve fibers, terminals, and vesicles, or if two or more peptides are co-contained in these compartments. We shall study them first in the internal layer of the median eminence, a region in the hypothalamus at the base of the brain where they are highly concentrated. Some of the neuropeptides in this layer are transported to the posterior pituitary, where they are known to have important neuroendocrine functions. An understanding of their ultrastructural relationships in the median eminence is needed to further our knowledge of their role. Our longer term objective is to study the ultrastructural distribution of neuropeptides in the striatum, globus pallidus, and substantia nigra. Neurological disorders such as Parkinson's disease and Huntington's chorea are associated with abnormalities in these nuclei, and a knowledge of how neuropeptides are compartmentalized in them may prove important in understanding and treating these diseases.